The present invention relates to a fuel cell having two electrodes of different polarity, between which an electrolyte is provided. The electrode has pores, which are permeable to a fluid, in particular a gas. The pores have pore inlet openings and pore outlet openings and are permeable from a gas-side surface to an electrolyte-side surface.
In fuel cells, just like in other galvanic cells and electrochemical elements, the bonding energy released, for example, when hydrogen (H2) chemically combines with oxygen (O2) is converted into electrical energy and heat. A fundamental distinction is made between low-temperature fuel cells (up to about 200xc2x0 C.) and high-temperature fuel cells (about 600 to 1100xc2x0 C.). In between the low-temperature fuel cells and the high-temperature fuel cells, there are so-called molten carbonate fuel cells (MCFCs) with an operating temperature from about 200 to 600xc2x0 C. and with a liquid electrolyte provided in a matrix.
High-temperature fuel cells, such as oxide-ceramic fuel cells (SOFC; Solid Oxide Fuel Cell) contain, for example, a solid electrolyte composed of zirconium dioxide which conducts ions at an operating temperature of 850 to 1050xc2x0 C. These are mainly used for local, decentralized power supply systems, particularly in stationary installations.
In conjunction with an electric motor, low-temperature fuel cells could represent an alternative to conventional internal-combustion engines, particularly in vehicles and railroad systems.
In known electrical vehicles, the electrical energy is initially produced in a power station and is then temporarily stored in a battery in the vehicle. High costs, heavy weight, limited life and long charging times for these batteries are problems which have not yet been solved satisfactorily.
Concepts which produce the electricity whenever it is required in the vehicle, and manage without a temporary storage therefore appear particularly promising, in particular the concept of fuel cells with a proton-conducting membrane electrolyte, so-called proton exchange membrane fuel cells (PEM fuel cells). The gaseous fuel, in particular hydrogen gas and oxygen gas, need not be burnt, but is converted directly to electrical energy and steam in a so-called cold reaction. The electrolyte in the PEM fuel cell separates the two gases from one another and prevents a so-called hot reaction. An electrochemical process on the electrolyte allows only protons, that is to say positively charged hydrogen ions (H+), to pass. The electrons of the hydrogen atoms are separated out during this passage and remain behind, and the hydrogen ions react with the oxygen particles on the other side. Excess electrons on the hydrogen side and a lack of electrons on the oxygen side of the electrolyte result in a potential difference between the adjacent electrodes so that, if the electrodes are electrically connected via an external circuit in which a load is connected, an electrical current flows from the anode to the cathode. Apart from the electrical energy, heat and water are produced as reaction products.
The electrolyte in such PEM fuel cells includes a proton-conducting polymer film which is only a few tenths of a millimeter thick. The electrodes are coated with a catalyst containing platinum. However, the polymer film may also alternatively and/or in addition be coated with at least one catalyst on both sides. The catalyst promotes ionization of the hydrogen and the reaction of the hydrogen ions with the oxygen.
The other fuel cell types described initially contain, for example, catalysts composed of Raney nickel, tungsten carbide, molybdenum sulfides, tungsten sulfides or phthalocyanin complexes and other chelate complexes.
Hydrocarbons, in particular methanol and methane, are also used as fuels, alternatively and/or in addition to hydrogen (H2).
In low-temperature fuel cells, in particular PEM fuel cells, the surfaces of the electrolyte and/or the electrolyte-side surfaces of the adjacent electrodes must be kept moist in order to promote the reaction and achieve good efficiencies. To achieve this, it is known, for example, from German Patent No. DE 43 18 818 C2 to operate the fuel cell with moistened gases.
In a stack of series-connected fuel cells, so-called bipolar plates are provided between adjacent fuel cells. These carry the fuel, in particular hydrogen, and respectively carry the air along the gas-side surfaces of the electrodes that is along the inlet openings of the pores in a labyrinthine channel system. Furthermore, they dissipate the reaction heat and provide the electrical connection to the adjacent cell.
The gas, which is supplied at normal pressure or high pressure depending on the type of fuel cell, diffuses through the porous electrode and reacts with the electrolyte, assisted by a catalyst, as described.
A high-power fuel cell system is produced by interconnecting a large number of fuel cells in so-called stacks. The energy output from the system can be produced directly, and controlled precisely, by controlling the supply of hydrogen.
It is accordingly an object of the invention to provide a fuel cell which overcomes the disadvantages of the heretofore-known fuel cells of this general type and which has an electrode with an improved permeability and porosity for a fluid, in particular for a gas.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel cell, including:
two electrodes of different polarity;
a proton-conducting membrane electrolyte disposed between the two electrodes;
at least one of the two electrodes having a gas-side surface and an electrolyte-side surface;
the at least one of the two electrodes being formed with pores, the pores being permeable for a gas from the gas-side surface to the electrolyte-side surface and the pores having respective pore inlet openings and pore outlet openings; and
the pore inlet openings have a first total area and the pore outlet openings have a second total area smaller than the first total area.
The electrode of the fuel cell according to the invention is distinguished by the fact that the total area of the pore inlet openings is larger than the total area of the pore outlet openings. This advantageously results in better permeability of the electrode for a fluid, in particular for a gas, and, furthermore, reduces the moisture loss from the layer which is in contact with the electrolyte and the electrolyte-side surface of the electrode.
According to a preferred embodiment of the fuel cell electrode, the total area ratio of pore inlet openings to pore outlet openings is at least 2:1, preferably at least 3:1, and in particular about 4:1. Such a total area ratio particularly assists gas diffusion through the porous electrode.
The pores are preferably at least partially formed in the form of funnels, so that the pores taper from the inlet opening toward the outlet opening.
Alternatively and/or in addition, the pores are at least partially stepped, so that the pore inlet openings have a larger cross section than the pore outlet openings. Such a stepped configuration includes at least a single stepping within the pores and can preferably be achieved, according to the invention, for example, by providing the electrode with at least two layers, namely a gas-side layer and an electrolyte-side layer, wherein the gas-side layer predominantly has pores with a larger cross section than that of the pores in the electrolyte-side layer. Electrodes having a layered construction can be produced easily and cost-effectively.
Electrodes of fuel cells according to the invention which have such a pore structure preferably contain graphite, or are at least partially composed of graphite.
Electrodes which are coated with at least one catalyst are preferably configured such that the catalyst is provided predominantly adjacent to the electrolyte-side surface, preferably in the electrolyte-side pore cross sections. Such a configuration firstly allows to considerably minimize the quantity of catalyst used. The catalyst preferably contains platinum. As a result, the fuel cell can be produced more cost-effectively. Secondly, the catalyst is advantageously provided precisely where it promotes and assists the reactions already described above.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fuel cell having an electrode with pores which are permeable for a fluid, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.